L0301P1 - Body Fluids
__TOC__ Water *H2O = 55.55…M **much higher than the concentration of any particle in the human body ***Na+plasma = 140 mM ***H+ecf = 40 nM *self-ionisation is a function of temperature Water in the Body *2/3 in the intracellular fluid - ICF (maintains size of the cell) *1/3 in the extracellular fluid - ECF (circulation, between your cells) Definitions Solvent *the liquid in which substances are dissolved *almost always water in human body Solute *the dissolved substance Infinite Bath *1L bath of solution in which a red blood cell (RBC) is placed *a RBC is so small that it is basically negligible - the concentration of the bath is considered as constant Osmolarity *number of particles per volume *mole/L H2O *unit: Osmoles (Osm) Osmolality *number of particles per weight *mole/kg H2O *osmolarity = osmolality Diffusion *occurs down the concentration gradient **“down d[ ]/dy” *due to thermal properties of matter **i.e. no energy is required Osmotic Pressure *if particles are permeable **the solute and water spread evenly across the volume *if the particles are not permeable **more particles pushing against the membrane on the left than the right **causing the water to move into the left of the tank *the pressure placed by a piston to just stop the water from moving across the membrane (from right to left) = osmotic pressure *a.k.a tonicity - tonoles Effective vs Ineffective Osmoles Effective Osmoles *cannot pass through the cell membrane *require special channels and/or transporters to enter a cell *creates osmolarity difference/concentration gradient and therefore results in osmosis occurring *eg. Many ions are effective osmoles, eg K+, Na+, Cl- Ineffective Osmoles *can pass through the cell membrane *will simply diffuse through the cell membrane to make the concentration of particles (osmolarity) equal Tonicity *a.k.a effective osmolarity *similar to osmolarity, but only deals with effective osmoles *the number of moles of particles that cannot pass though a cell membrane per litre Common Prefixes *seen in terms used for comparing the osmolarity of two solutions, or concentrations of particles in each solution Hyper *more than Hyperosmolar *solution A has a higher concentration of particles than solution B *therefore solution A is hyperosmolar to solution B Hypertonic *solution A has a higher concentration of effective osmoles than solution B *solution A has a higher osmotic pressure than solution B *therefore solution A is hypertonic to solution B Iso *equal to Iso-osmolar *solution A has the same concentration of particles as solution B *therefore solution A is hyperosmolar to solution B Isotonic *solution A has the same concentration of effective osmoles as solution B *solution A has the same osmotic pressure as solution B *therefore solution A is hypertonic to solution B Hypo *less than Hyperosmolar *solution A has a lower concentration of particles than solution B *therefore solution A is hyperosmolar to solution B Hypertonic *solution A has a lower concentration of effective osmoles than solution B *solution A has a lower osmotic pressure than solution B *therefore solution A is hypertonic to solution B Experiments *a red blood cell is to be placed in an infinite bath *based on the osmolarity of all involved molecules and their diffusibility (permeable to the membrane), what happens to the RBC can be predicted *note that: a RBC concentration is approximated to 300 mOsm/L (milliosmoles per litre) Molecule Permeability *sodium (Na+) - effectively impermeable to the cell membrane *urea - permeable to the cell membrane *dextrose (dx) - impermeable to the cell membrane for a while Case 1. Pure Water Osmolarity *Bath - 0 *Cell - 300 mOsm/L Diffusibility *Bath - yes *Cell contents - no Results *water moves into the cell *RBC burst (a.k.a haemolysis) Case 2. Normal Saline *Normal saline that is used in medicine has a concentration of 0.90% w/v (300 mOsm/L) of NaCl in water Osmolarity *Bath - 300 *Cell - 300 Diffusibility *Bath - no since Na+ is impermeable *Cell contents - no Results *nothing will happen to the red blood cell as there is no concentration gradient Case 3. Iso-osmolar Urea Osmolarity *Bath - 300 *Cell - 300 Diffusibility *Bath - yes since urea is permeable to the cell membrane *Cell contents - no Results *urea will diffuse into the cell until the concentration of urea inside the cell is the same as the bath due to the fact the cell’s volume is so small *the osmolarity's when then become: **bath = 300 **cell = 300 + 300 urea = 600/L *water will then diffuse into the cell causing lysis (bursting) of the cell Case 4. 5% Dextrose *dextrose is a form of glucose *used to make a pure water solution iso-osmolar to the cell i.e. of the same osmolarity as the cells. *can be infused without causing RBC to lyse as it is isotonic. *dextrose will eventually enter the cell and within ~1 hour, is metabolised to CO2 and H2O **therefore infusing 5% dextrose is like infusing pure water and can be used to increase water volume in the body Other Notes * ISO-OSMOLAR ≠ ISOTONIC **infusion of isotonic NaCl expands the extracellular fluid and the intravascular space, whereas the infusion of 5% Dextrose expands the total body water (eventually)